diff --git a/pandas/core/groupby/grouper.py b/pandas/core/groupby/grouper.py
index 3bffe59905a69..3869c30374bcf 100644
--- a/pandas/core/groupby/grouper.py
+++ b/pandas/core/groupby/grouper.py
@@ -263,7 +263,7 @@ class Grouper:
     _gpr_index: Index | None
     _grouper: Index | None
 
-    _attributes: tuple[str, ...] = ("key", "level", "freq", "axis", "sort")
+    _attributes: tuple[str, ...] = ("key", "level", "freq", "axis", "sort", "dropna")
 
     def __new__(cls, *args, **kwargs):
         if kwargs.get("freq") is not None:
@@ -287,6 +287,7 @@ def __init__(
         self.freq = freq
         self.axis = axis
         self.sort = sort
+        self.dropna = dropna
 
         self.grouper = None
         self._gpr_index = None
@@ -295,7 +296,6 @@ def __init__(
         self.binner = None
         self._grouper = None
         self._indexer = None
-        self.dropna = dropna
 
     @final
     @property
@@ -339,7 +339,7 @@ def _get_grouper(
         return self.binner, self.grouper, self.obj  # type: ignore[return-value]
 
     @final
-    def _set_grouper(self, obj: NDFrame, sort: bool = False):
+    def _set_grouper(self, obj: NDFrame, sort: bool = False) -> None:
         """
         given an object and the specifications, setup the internal grouper
         for this particular specification
@@ -413,7 +413,6 @@ def _set_grouper(self, obj: NDFrame, sort: bool = False):
         # "NDFrameT", variable has type "None")
         self.obj = obj  # type: ignore[assignment]
         self._gpr_index = ax
-        return self._gpr_index
 
     @final
     @property
@@ -619,7 +618,7 @@ def codes(self) -> npt.NDArray[np.signedinteger]:
             # _codes is set in __init__ for MultiIndex cases
             return self._codes
 
-        return self._codes_and_uniques[0]
+        return self._factorize[0]
 
     @cache_readonly
     def group_arraylike(self) -> ArrayLike:
@@ -635,7 +634,7 @@ def group_arraylike(self) -> ArrayLike:
             # retain dtype for categories, including unobserved ones
             return self.result_index._values
 
-        return self._codes_and_uniques[1]
+        return self._factorize[1]
 
     @cache_readonly
     def result_index(self) -> Index:
@@ -653,11 +652,12 @@ def group_index(self) -> Index:
             # _group_index is set in __init__ for MultiIndex cases
             return self._group_index
 
-        uniques = self._codes_and_uniques[1]
+        uniques = self._factorize[1]
         return Index._with_infer(uniques, name=self.name)
 
     @cache_readonly
-    def _codes_and_uniques(self) -> tuple[npt.NDArray[np.signedinteger], ArrayLike]:
+    def _factorize(self) -> tuple[npt.NDArray[np.signedinteger], ArrayLike]:
+        """Analogous to core.algorithms.factorize"""
         if self._passed_categorical:
             # we make a CategoricalIndex out of the cat grouper
             # preserving the categories / ordered attributes
diff --git a/pandas/core/groupby/ops.py b/pandas/core/groupby/ops.py
index e4e42e7a1178e..c9381652da970 100644
--- a/pandas/core/groupby/ops.py
+++ b/pandas/core/groupby/ops.py
@@ -878,7 +878,7 @@ def has_dropped_na(self) -> bool:
 
     @cache_readonly
     def group_info(self) -> tuple[npt.NDArray[np.intp], npt.NDArray[np.intp], int]:
-        comp_ids, obs_group_ids = self._get_compressed_codes()
+        comp_ids, obs_group_ids = self._factorize()
 
         ngroups = len(obs_group_ids)
         comp_ids = ensure_platform_int(comp_ids)
@@ -899,10 +899,10 @@ def codes_info(self) -> npt.NDArray[np.intp]:
         return ids
 
     @final
-    def _get_compressed_codes(
+    def _factorize(
         self,
     ) -> tuple[npt.NDArray[np.signedinteger], npt.NDArray[np.intp]]:
-        # The first returned ndarray may have any signed integer dtype
+        """Analogous to core.algorithms.factorize"""
         if len(self.groupings) > 1:
             group_index = get_group_index(self.codes, self.shape, sort=True, xnull=True)
             return compress_group_index(group_index, sort=self._sort)
diff --git a/pandas/core/resample.py b/pandas/core/resample.py
index 53d75255da536..b6a0c988fa69a 100644
--- a/pandas/core/resample.py
+++ b/pandas/core/resample.py
@@ -1466,14 +1466,14 @@ def __init__(
         self,
         freq="Min",
         closed: Literal["left", "right"] | None = None,
-        label: str | None = None,
+        label: Literal["left", "right"] | None = None,
         how="mean",
         axis=0,
         fill_method=None,
         limit=None,
         loffset=None,
         kind: str | None = None,
-        convention: str | None = None,
+        convention: Literal["start", "end", "e", "s"] | None = None,
         base: int | None = None,
         origin: str | TimestampConvertibleTypes = "start_day",
         offset: TimedeltaConvertibleTypes | None = None,
@@ -1519,8 +1519,7 @@ def __init__(
         self.label = label
         self.kind = kind
 
-        self.convention = convention or "E"
-        self.convention = self.convention.lower()
+        self.convention = convention or "e"
 
         self.how = how
         self.fill_method = fill_method